Known is an inverted pendulum type vehicle or an omni-directional vehicle comprising a pair of drive assemblies individually actuated by electric motors and a main wheel held between the drive assemblies and frictionally driven by the drive assemblies. See WO2008132779A1 (US20100096905A1) (patent document 1) for instance. Each drive assembly comprises a drive disk coaxially opposing the drive disk of the other drive assembly and a plurality of drive rollers obliquely arranged along the circumference of the drive disk at a regular interval so as to be individually rotatable. The main wheel comprises a ring-shaped annular member rotatably supported by a frame around a central axial line thereof and a plurality of driven rollers arranged along the circumference of the annular member so as to be rotatable around the respective tangential lines. As the drive disks are turned by the electric motors, the driven rollers are frictionally driven by the drive rollers. When the drive rollers are turned around the tangential directions of the main wheel, the vehicle is driven in a lateral direction. When the main wheel is turned around the central axial line thereof, the vehicle is driven in a fore and aft direction. The direction of motion of the vehicle can be selected as desired by suitably adjusting the difference between the rotational speeds of the two drive disks.
The frictional drive device for this vehicle requires the two drive disks to be rotatively actuated individually, and electric motors are provided individually for the two drive disks. In the previously proposed frictional drive device, the motors are provided radially spaced away from the corresponding drive disks, and power transmission devices such as those using endless belts were required to transmit the power of the electric motors to the corresponding drive disks.
Therefore, the frictional drive device was highly complex in structure, and difficult to maintain owing to poor accessibility of various parts. Also, the need for the power transmission devices prevented a compact design of the frictional drive device.
The frictional drive device of this type requires a plurality of rollers to be mounted on each drive disk in a specific orientation. In the previous proposal, each drive roller is mounted on a bracket via a roller shaft, and the bracket was in turn fixedly secured to a peripheral part of the drive disk. Therefore, a large number of brackets were required, and this necessitated a large number of component parts and many work hours of assembly work. Also, special jigs were required for the assembly work, and this impaired the efficiency of the assembly work, and increased the manufacturing cost. Furthermore, the need for the brackets impaired the assembling precision, and this reduced the performance or efficiency of the frictional drive device.
The vehicles of this type are suited to have a small footprint (area of the image of the vehicle projected on the ground surface), and this enables the vehicle to travel in narrow spaces. Japanese patent laid open publication No. 2006-282160 (patent document 2) discloses such an inverted pendulum type vehicle comprising a robot main body consisting of a spherical wheel and an omni-directional drive unit for rolling the spherical wheel in a desired direction, and a control unit for maintaining the robot main body in an upright posture under an inverted pendulum control.
In this vehicle, the robot main body further includes a casing receiving a battery, a control computer, motor drivers, a gyro sensors and other control components, and a protective cover attached to a lower part of the casing and covers the omni-directional drive unit. When this structure is applied to an inverted pendulum vehicle using a main wheel having a relatively small width, the protective cover may have a small width, but the casing is required to have a large width that does not match the narrow width of the protective cover. Therefore, the vehicle is prevented from having a small footprint.